The present invention relates to an AM/FM band selector that can select AM/FM bands by means of a variable condensor resonance electronic switch. In particular, an electronic muting AM/FM band selector is presented that can eliminate impulse noise (pop-noise) occurring in the cut-off of a power supply or in the selection of an AM/FM band.
According to the conventional art as shown in FIG. 3, outputs of an AM receiver 31 and an FM receiver 32 are connected by analog switches 33, 34, 35, and 36 to outputs L and R, respectively. On the other hand, the power supply to the AM receiver 31 or the FM receiver 32 is selected by switches SW1 and SW2 which are connected to a flip flop 37 and includes NAND gates G1 and G2. While a capacitor C1 is charged upon an initial application of the power supply B.sup.+ by the source voltage, the outputs of inverters I1 and I2 become respectively "high" and "low" until the charge voltage reaches an operational voltage V.sub.TH of the inverter 1. Subsequently, the NAND gate G1 receives a "low" input and the NAND gate G2 receives a "high" input which causes the outputs of the NAND gates G1 and G2 to become "high" and "low", respectively. As a result, transistor Q5 becomes conductive so that transistor Q6 is turned OFF and transistor Q3 is not conductive so that transistor Q4 supplies power from B.sup.+ to the FM receiver 32. Thus, the FM receiver 32 is selected upon the initial application of power. Simultaneously, the output from the inverter I1 and the output from the NAND gate G2 become "high" through diode D5 so that the transistor Q2 is turned ON and lights a light-emitting diode LD2 to indicate the IN-OPERATION of the FM receiver 32. While the output of the NAND gate G2 becomes "low" and the transistor Q3 is turned OFF, the transistor Q4 is turned ON to apply the power source B.sup.+ to FM receiver 32. Furthermore, transistors Q9 and Q10 are conductive for eliminating impulse voltage and accordingly, impulse noise (pop-noise) at the initial application of the source B.sup.+ is eliminated.
Upon completing the charging of capacitor C1 after the initial operation, one input of the NAND gate G2 becomes "high" from the outputs of inverters I1 and I2 and the other input of the NAND gate G2 also becomes "high" to maintain the outputs of the NAND gates G1 and G2. The output of inverter I1 becomes "low" and is applied through diode D5 to the bases of transistors Q9 and Q10 so that transistors Q9 and Q10 are turned OFF. Thereby, the output of the FM receiver 32 is delayed for the charging time of the capacitor C1 which is applied through analog switches 35 and 36. Additionally, upon pressing the switch SW1 for switching FM/AM bands, the output state of the flip flop 37 changes and transistor Q7 becomes conductive at the moment of the state change so that the output of inverter I1 becomes "low" and transistors Q9 and Q10 conduct. Thus, the drive voltage at the control terminal of the analog switch is lowered and removes impulse noise in band switching. When the charging voltage of capacitor C1 reverses to less than the operational voltage V.sub.TH upon the cutting-off of the power supply, a low signal is applied to the output of NAND gates G1 and G2 for turning off the analog switch and removing impulse noise.